The decade after World War Two saw communism spreading to the Far East. While China became communist in 1949, it was the conflict in Korea that proved to be the biggest crisis for relations between the USA and the USSR in the 1950s.
The success of communism in China had persuaded the USA that their domino theory was correct. This suggested that if one country was allowed to fall to communism then communism could quickly spread to neighbouring countries. In 1950 a report by the American National Security Council recommended that the policy of containment was not enough, and that what was needed was ‘roll back’ or action to take back territory from communist control.
After World War Two, Korea had been divided at the 38th parallel into the Soviet-backed communist North Korea, led by Kim Il Sung, and non-communist, American-backed South Korea under the leadership of Syngman Rhee.
In June 1950, with the support of China and the Soviet Union, North Korea launched an attack on South Korea across the 38th parallel.
The Korean War was an important development in the Cold War because it was the first time that the two superpowers, the United States and the Soviet Union, had fought a ‘proxy war’ in a third country. The proxy war or 'limited war' strategy would be a feature of other Cold War conflicts, for example the Vietnam War. It also established a precedent of the USA being involved in events in Asia.
Even when the fighting had stopped, American soldiers remained stationed in South Korea which was a worry for the Chinese and put pressure on relations between the two countries.